In a system of a precharging circuit having a large capacitor (such as a bus capacitor), when a precharging relay is at fault to be unable to be closed, there will emerge a problem that a precharging resistor become overheated.
For example, FIG. 1 shows a frequency converter having a precharging circuit.
As shown in FIG. 1, when precharging relay S1 can not be closed, during the period of charging and discharging of capacitor C1, due to voltage dividing of resistor R1, resistor R1 will become overheated, and capacitor C1 will have undervoltage due voltage dividing of resistor R1.
A product usually needs to be subject to UL (Underwrites Laboratories Inc) tests in order to meet security testing requirements. For the system of a precharging circuit of a large capacitance involved in the present application, the UL testing requirements are as follows:
1) the precharging relay is never closed (i.e., manually opening the precharging relay, and simulating a fault situation) and the motor is under a full load. All load currents will pass the precharging resistor.
2) when the precharging resistor becomes overheated, it is checked the cotton-like object around the frequency converter will not be burned so as to ensure the security of the customer.
When the motor runs under a rated load and at a rated speed, the precharging relay is opened again. A monitor for monitoring the voltage of a main circuit indicates “NOFF” (i.e., undervoltage in the main circuit). Next, the frequency converter runs again and “NOFF” occurs again.
After about 15 minutes, due to overheating of the precharging resistor, the precharging resistor always ignites the cotton-like object, but the precharging resistor is not fused.
FIG. 2 shows a device which is burned and damaged due to overheating of a precharging resistor. As shown in FIG. 2, the whole device is damaged since the precharging resistor ignites the cotton-like object.
When conducting this kind of test, before the burning appears, the precharging resistor will be damaged (broken circuit), and there is no other protection method.
This kind of method has the following dangers:                There also exists the risk of ignition, and the resistor is damaged so that it is not very reliable; and        When this case appears, the frequency converter will be damaged permanently.        